The invention lies in the circuit technology and electronics fields. More specifically, the invention relates to a circuit configuration for mixing an input signal and an oscillator signal with one another.
Mirror frequency suppression mixers are an advantageous solution for saving complex mirror frequency suppression filters in input receivers and for achieving greater integration. However, such mixers have the disadvantage that they require more than twice the power of a conventional mixer. The question of power consumption is of major importance in particular for mobile applications, since the power consumption in that case has a major influence particularly on the costs involved, the construction of the respective appliance, and its characteristics.
By way of example, signal frequency suppression mixers are known from U.S. Pat. Nos. 4,801,900 and 5,661,485. As a rule, such mixers comprise a low-noise input amplifier, two identical mixing stages, each having an amplifier connected to them for an oscillator signal, a phase splitter for producing two orthogonal oscillator signals from the original oscillator signal, and an output phase combiner. In that case, the input and output impedance ratios play a particular role. A more or less high input impedance for the amplifiers for the oscillator signal normally means increased power consumption. Furthermore, the insertion loss resulting from these additional elements must be compensated for by the phase combiner at the output, which once again results in increased power consumption. Conventional low-noise amplifiers are constructed as power/voltage converters with an input-side transistor (or transistor pair in a differential configuration) connected in cascode, which drives a passive load such as a parallel low-pass filter network with resistors and capacitors. Voltage/current feedback by means of a resistor requires a precisely matched input impedance in order to achieve the required linearity. Although inputs with common emitters are most suitable for low noise, they need to be operated with a very high bias current (several milliamperes), however. The power signal at the input is thus converted to a current which drives the passive load and leads to a voltage as the output signal at the output. However, large voltage signals require a corresponding DC voltage drive level margin.
Furthermore, it is necessary for the mixers to have a considerably higher input impedance than the output impedance of the amplifiers, in order to reduce the insertion loss. An input stage with a common emitter as in the case of an amplifier must be implemented in this case, and this must be subject to additional linearity requirements, owing to the amplifier gain. This linearity can be achieved by emitter degeneration, but this means that the mixer must have a very high output impedance in order to achieve a voltage converter is gain of approximately 0 dB. For its part, this once again means that a greater drive level margin is required. The phase combiner is also subject to the same requirements as well as requiring an additional drive level margin. Thus, if the blocks are placed one on top of the other then, although the current consumption is lower, a considerable drive level margin is required, however, which does not allow operation at a low voltage, as is required for mobile applications (for example 2.7 V). On the other hand, circuit configurations in which the blocks are arranged in parallel have a far greater power consumption with otherwise poorer characteristics.
It is accordingly an object of the invention to provide a circuit configuration for mixing an input signal with an oscillator signal, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for mixing an input signal and an oscillator signal with one another, comprising:
a phase splitter having an input receiving an oscillator signal, and first and second voltage outputs carrying orthogonal oscillator signals, phase-shifted through 90xc2x0 with respect to one another;
a first differential amplifier having a voltage input connected to the first voltage output of the phase splitter and a current output;
a second differential amplifier having a voltage input connected to the second voltage output of the phase splitter and a current output;
a first controllable current source connected to and supplying the first differential amplifier, the first controllable current source receiving and being controlled by an input signal;
a second controllable current source connected to and supplying the second differential amplifier, the second controllable current source receiving and being controlled by the input signal;
a first phase shifter connected downstream of the first differential amplifier in a signal flow direction, the first phase shifter having a current input and a current output;
a second phase shifter connected downstream of the second differential amplifier in the signal flow direction, the second phase shifter having a current input and a current output; and
an adder device connected downstream from the first and second differential amplifiers and generating an output signal.
The present invention allows individual circuit blocks to be stacked one on top of the other in such a manner that, firstly, the bias currents can be divided by the individual blocks, and such that, on the other hand, they can themselves operate at very low supply voltages down to 2.7 V. The mixers in this case have a very low input impedance (owing to the coupled bases), and are each fed with half the current (at half the bias current level of the low-noise amplifier). Furthermore, such architecture means that only a low voltage drive level margin is required. Since the amplifiers which amplify the oscillator signal are designed such that the output impedance is sufficiently low, the mixer can be reduced to a simple switch pair which switches the oscillator signal and replaces the input stage with a common base. The phase combiner, which is constructed in a similar manner, can then, finally, also be placed on the mixer and receive the same bias current as the other circuit parts. Since the bias current levels are high, the input impedances of all the blocks are sufficiently low that a low signal loss occurs overall. The overall gain is essentially produced by the low-noise amplifier in order advantageously to compensate for higher noise levels in the mixers as a result of the signal being split at its input. Finally, stacking the phase combiner and the mixer one on top of the other allows the power consumption of the overall arrangement to be reduced considerably.
In accordance with an added feature of the invention, the first and second phase shifters each has one balanced input and one balanced output, each having one inverting and one non-inverting connection; and wherein the inverting connection of the input, the non-inverting connection of the input, the inverting connection of the output, and the non-inverting connection of the output are directly connected and cross-connected via respective two resistors and two capacitors. In other words, the two phase shifters each have one balanced input and one balanced output, each having one inverting and one non-inverting connection. In this case, the input-side inverting and non-inverting connections are respectively connected to the output-side inverting and non-inverting connections, directly and crossed over by means of two resistors and two capacitors in each case. This allows a suitable phase shifter current input and output to be provided with little circuitry complexity and using passive circuit technology. The passive implementation furthermore has the advantage that no significant additional noise is produced, as in the case of active phase shifters.
In accordance with an additional feature of the invention, the controllable current sources each have:
a first constant current source connected in series with the respective the differential amplifier;
a second constant current source connected in parallel with the respective the differential amplifier; and
an amplifier stage connected in parallel with the first constant current sources.
In other words, the controllable current sources preferably have a first constant current source connected in series with the respective differential amplifier, a second constant current source connected in parallel with the respective differential amplifier, and an amplifier stage connected in parallel with the first constant current sources.
In accordance with a concomitant feature of the invention, a cascode circuit is preferably provided for the amplifier stage and/or the adder device and, in particular, is fed from a common reference voltage source.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for mixing an input signal and an oscillator signal with one another, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.